This invention relates generally to radio frequency antenna beam forming networks and more particularly to active beam forming networks adapted for use in connection with a multibeam antenna system to couple a selected one or ones of the antenna feed elements in the antenna system to receiver/transmitter apparatus thereby to form a desired composite antenna beam.
One application for such active beam forming network is in a satellite communication system. Here a geo-stationary satellite in a synchronous circular equatorial orbit above the earth carries a transponder to enable radio frequency communication between locations on the earth's surface. The transponder carried in the satellite in such application transmits and receives signals using a multibeam antenna system. The multibeam antenna system is coupled to a beam forming network which directs an antenna beam in accordance with command signals transmitted to the satellite from a station on the earth. In the particular satellite communication system herein described, the multibeam antenna system is configured so that each one of a plurality of "spot" beams covers a different, slightly overlapping area on the earth's surface viewed by the satellite. Each one of the "spot" beams is formed from an aperture having, substantially, the gain and bandwidth of the whole antenna. The beam forming network may be used on both transmit and receive to form selected one or ones of the plurality of "spot" beams. The antenna beam could vary anywhere from a single "spot" beam for spot coverage (i.e. as to cover the east coast of the United States for example) to a cluster of "spot" beams capable of covering the entire earth's surface as viewed by the satellite. Further, the beam forming network may be used to couple selected "spot" beams in particular amplitude and phase relationship in order to generate nulls in the antenna beam at angles corresponding to the locations of various jammers which may be present on the earth's surface.
One known beam forming network which is adapted for the application described above includes a metal plate lens and a planar array of n antenna feed elements disposed in the focal plane of the metal plate lens, each one of such antenna feed elements being coupled to a receiver/transmitter through a "tree" network of variable power dividers. The "tree" network has at its apex a single variable power divider having an input coupled to the receiver/transmitter. Each one of a pair of output ports of such power divider is coupled to a corresponding one of two different variable power dividers which, in turn, are coupled to four different power dividers. The "tree" network continues until n/2 variable power dividers are coupled to the n antenna feed elements. The power division ratio of each one of the variable power dividers in the "tree" network is individually set by control signals supplied thereto by the command signals from the station on the earth to synthesize the proper amplitude distribution over the array of antenna feed elements and thereby develop the desired composite antenna beam. Included in the path of each one of the antenna feed elements is a corresponding variable phase shifter which is also set by the command signals to provide a proper phase distribution and thereby develop the desired antenna beam. Preamplifiers are also disposed in the paths of the antenna feed elements to serve as a buffer against insertion losses associated with the beam forming network. While such beam forming network may generate the desired antenna beam, it is noted that failure of a single preamplifier, phase shifter or power divider may result in complete loss of coverage in the associated "spot" beam and, in fact, failure of a single critical variable power divider, specifically that which is connected to the receiver/transmitter at the apex of the "tree" network, results in the complete loss of half of the maximum possible antenna beam coverage. In the satellite application described above replacement of such preamplifier phase shifter or power divider is not practical for obvious reasons and hence the overall performance of the system must be designed to ensure high reliability over many years.